Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
A CDMA spread spectrum (“SS”) signal is created by modulating the radio frequency (“RE”) signal with a spreading sequence (a code consisting of a series of binary pulses) known as a pseudo-noise (“PN”) digital signal because they make the signal appear wide band and “noise like”. The PN code runs at a higher rate than the RF signal and determines the actual transmission bandwidth. The resulting signal has a low-power spectral density in any narrow portion of the band. Messages can be cryptographically encoded to any level of secrecy desired with direct sequencing, as the entire transmitted/received message is purely digital.
The spreading sequence is a long sequence of binary pulses or bits that does not repeat itself. It is, therefore, often referred to as a long code. Variations of this code include a convolutional code, a turbo code, or a Walsh code, as is well known to those of ordinary skill in the art.
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication systems as described in Provisional U.S. Patent Application No. 60/082,671, filed Apr. 22, 1998, and incorporated herein by reference. These WCDMA systems are coherent communications systems with pilot symbol assisted channel estimation schemes. These pilot symbols are transmitted as quadrature phase shift keyed (QPSK) known data in predetermined time frames to any receivers within range. The frames may propagate in a discontinuous transmission (DTX) mode. For voice traffic, transmission of user data occurs when the user speaks, but no data symbol transmission occurs when the user is silent. Similarly for packet data, the user data may be transmitted only when packets are ready to be sent. The frames include pilot symbols as well as other control symbols such as transmit power control (TPC) symbols and rate information (RI) symbols.
These control symbols include multiple bits otherwise known as chips to distinguish them from data bits. The chip transmission time (TC), therefore, is equal to the symbol time rate (T) divided by the number of chips in the symbol (N).
Previous studies have shown that multiple transmit antennas may improve reception by increasing transmit diversity for narrow band communication systems. In their paper New Detection Schemes for Transmit Diversity with no Channel Estimation, Tarokh et al. describe such a transmit diversity scheme for a TDMA system. The same concept is described in A Simple Transmitter Diversity Technique for Wireless Communications by Alamouti. Tarokh et al. and Alamouti, however, fail to teach such a transmit diversity scheme for a WCDMA communication system.
Other studies have investigated open loop transmit diversity schemes such as orthogonal transmit diversity (OTD) and time switched time diversity (TSTD) for WCDMA systems. Both OTD and TSTD systems have similar performance. Both use multiple transmit antennas to provide some diversity against fading, particularly at low Doppler rates and when there are insufficient paths for the rake receiver. Both OTD and TSTD systems, however, fail to exploit the extra path diversity that is possible for open loop systems. For example, the OTD encoder circuit of FIG. 5 receives symbols S1 and S2 on lead 500 and produces output signals on leads 504 and 506 for transmission by first and second antennas, respectively. These transmitted signals are received by a despreader input circuit (FIG. 6). The input circuit receives the ith of N chip signals per symbol together with noise along the jth of L multiple signal paths at a time τj after transmission. Both here and in the following text, noise terms are omitted for simplicity. This received signal rj (i+τj) at lead 600 is multiplied by a channel orthogonal code signal Cm (i+τj) that is unique to the receiver at lead 604. Each chip signal is summed over a respective symbol time by circuit 608 and produced as first and second output signals Rj1 and Rj2 on leads 612 and 614 as in equations [1–2], respectively. Delay circuit 610 provides a one-simbol delay T so that the output signals are produced simultaneously.
                              R          j          1                =                                            ∑                              i                =                0                                            N                -                1                                      ⁢                                          r                j                            ⁡                              (                                  i                  +                                      τ                    j                                                  )                                              =                                                    α                j                1                            ⁢                              S                1                                      +                                          α                j                2                            ⁢                              S                2                                                                        [        1        ]                                          R          j          2                =                                            ∑                              i                =                N                                                              2                  ⁢                  N                                -                1                                      ⁢                                          r                j                            ⁡                              (                                  i                  +                                      τ                    j                                                  )                                              =                                                    α                j                1                            ⁢                              S                1                                      -                                          α                j                2                            ⁢                              S                2                                                                        [        2        ]            
The OTD phase correction circuit of FIG. 7 receives the signals Rj1 and Rj2 as input signals corresponding to the jth of L multiple signal paths. The phase correction circuit produces soft outputs or signal estimates {tilde over (S)}1 and {tilde over (S)}2 for symbols S1 and S2 at leads 716 and 718 as shown in equations [3–4], respectively.
                                          S            _                    1                =                                            ∑                              j                =                1                            L                        ⁢                                          (                                                      R                    j                    1                                    +                                      R                    j                    2                                                  )                            ⁢                              α                j                                  1                  *                                                              =                                    ∑                              j                =                1                            L                        ⁢                          2              ⁢                                                                                      α                    j                    1                                                                    2                            ⁢                              S                1                                                                        [        3        ]                                                      S            _                    2                =                                            ∑                              j                =                1                            L                        ⁢                                          (                                                      R                    j                    1                                    -                                      R                    j                    2                                                  )                            ⁢                              α                j                                  2                  *                                                              =                                    ∑                              j                =                1                            L                        ⁢                          2              ⁢                                                                                      α                    j                    2                                                                    2                            ⁢                              S                2                                                                        [        4        ]            Equations [3–4] show that the OTD method provides a single channel estimate α for each path j. A similar analysis for the TSTD system yields the same result. The OTD and TSTD methods, therefore, are limited to a path diversity of L. This path diversity limitation fails to exploit the extra path diversity that is possible for open loop systems as will be explained in detail.